1. A cell line established from the spleen of a III/J strain inbred New Zealand white rabbit, inoculated with Herpesvirus saimiri (HVS) and a tumor promoting agent, has been characterized. The cells (7710) are of T origin, are HVS nonproducer, but have been shown by hybridization that they are HVS DNA positive. The cells grow in tissue culture either in large clumps or as single cells only in the presence of a T-cell growth factor. These cells consistently induced a malignant T-cell lymphoma when transplanted in other III/J inbred rabbits. 2. The ability of an in vitro replicated transfer factor against Herpesvirus saimiri to transfer the immunity in negative hosts has been studied in several owl and rhesus monkeys.